We know Legion raid bosses can drop legendary items and the chance remains in different difficulties. So you can go in Raid Finder, Normal, Heroic and Mythic to kill the same boss(es) 4 times for loot and you have 4 chances to get a legendary.
My question is different, though. Is it possible to get a legendary item from a raid boss you have already killed this week on that difficulty? You can use another bonus roll on that boss, but you can't loot normal items from it and legendary items are special personal loot, so this is somewhat tricky.
I've heard rumors about this being possible, but haven't seen this, yet. We even cleared Heroic Emerald Nightmare a couple of times when recent server lags made it hard to go Mythic, so my people were sure to get another shot at a legendary.
PS: This is only about raid bosses. Dungeon bosses grant loot every time you queue for them, so they always have their legendary drop chance.